1. Field of the Invention
The present invention relates generally to a system and method for locating moving objects in a field crowded with such objects. More particularly, the invention relates to determining location based on the transmission of reference signals from at least two spaced-apart locations, reception of the signals by each of the moving objects, and interpretation of the reference signals to determine the bearings of each of the transmission locations relative to the moving objects.
The ability to locate and track moving objects in a crowded field is of concern in a number of applications, including aircraft control, vehicular control in urban areas, and control of transportation vehicles in facilities such as open pit mines, warehouses, airports, and the like. Conventional systems, such as radar, usually rely on human observation and interpretation of a visual display. While effective in many circumstances, systems which require human interpretation suffer from a number of drawbacks. The number of objects which may be tracked is limited. The accuracy of the positional determinations is not high. Finally, the information discerned by the human operator is not easily stored and manipulated using computers and other automatic data processing techniques.
To supplement radar, a variety of navigational systems, such as LORAN A, LORAN C, OMEGA, and Decca, have been developed to assist in long range navigation of ships and planes. Such systems generally rely on broadcast of locational signals from a number of fixed points and determination of position based on conventional triangulation calculations. While such systems are very effective for providing navigational assistance, they do not generally allow for central compilation of locational data on a large number of planes and ships. Moreover, even when such systems are modified with transponders on the vehicles to provide for central collection of data, the systems are not highly accurate and are generally unsuitable for surface vehicular control in relatively small areas, such as individual cities, airports, and other facilities.
For the above reasons, it would be desirable to provide a method and system for locating the positions of moving objects with high accuracy, such as q1 foot or better. It is further desirable that such systems would be effective over relatively small areas, such as the area of a city or individual facility, and that the system could locate and track hundreds and even thousands of moving objects simultaneously. Finally, it would be desirable if the system were digitally controlled and able to interface with control systems whereby the control system would be capable of directly controlling the movement and positions of the various moving objects.
2. Description of the Background Art
U.S. Pat. No. Re. 31,962 describes a LORAN-C navigational system which utilizes a plurality of fixed broadcast stations which transmit synchronized, pulsed low-frequency signals. A ship or plane receives signals from at least three of the fixed stations, and can calculate position based on the phase shift between the signals.
U.S. Pat. No. 4,305,077 describes a navigational system which relies on only a single ground reference station which transmits three signals: a scanning signal, a positional information signal, and an azimuth information signal. Each vehicle includes receivers for the various signals, and the position may be calculated based on the time or phase shift between the receipt of the omnidirectional azimuth information signal and the unidirectional scanning signal.
U.S. Pat. No. 4,319,243 describes a system for controlling aircraft in the immediate locality of an airport. The system includes a focus beam transmitter and a focused receiver, each of which scans the field. Each aircraft or other airport vehicle includes a transponder which broadcasts only when interrogated by the focused beam. The position of each aircraft and vehicle is determined at the moment when the aircraft is interrogated by the focus beam and the transponder signal is received by the focus receiver. See also U.S. Pat. Nos. 3,922,677 and 4,533,871, which relate to systems for locating surface vehicles.
United Kingdom Pat. No. 1,575,397 of Standard Telephones and Cables, Ltd., describes a Radio Lighthouse for marine navigation. Therein a system is described for transmitting bearing information wherein frequency- and time-specific binary coded signals and amplitude modulated Morse Code groups are employed for identifying the transmitter and certain bearing information. The system relies on logic timers to decode information imbedded in the signal format. VHF signals are employed.
What is needed is a vehicle tracking system capable of accurately pinpointing a plurality of vehicles within a predefined space from a remote location.